Air-cored high-frequency transformer



Sept. 20, 1938. R. SCHMOOK 2,130,432

AIR-CORED HIGH FREQUENCY TRANSFORMER Filed Aug. 3, 1934 INVENTOR PUDOLF SCHMOO/f ATTORNEY Patented Sept. 20, 1938 UNITED STATES PATENT OFFICE AIR-CORED HIGH-FREQUENCY TRANS- FORMER Application August 3, 1934, Serial No. 738,198 In Germany August 3, 1933 3 Claims.

The present invention relates generally to coupling systems and more particularly to air-core transformers.

The object of the present invention is to devise an air-cored high-frequency transformer which distinguishes itself from the forms of constructions customary in the earlier art in that it operrates with an extremely low leakage flux so that it insures a very high degree of coupling.

In accordance with this invention the said outstanding advantages are obtained by shielding the spaces through which the magnetic fluxlines are to pass by means of a casing of high electric conductivity from the emergence of alter- 'l5 nating current stray fluxes, said casing being electrically subdivided so that it will not constitute a short-circuiting ring for the main flux.

The shielding effect as here secured may be explained in such a way that such stray flux lines 20 as may escape, occasion in the conducting casing eddy-currents which, in turn, set up counteractive magnetic fields which will act in opposition to the leakage fields.

T wo exemplified embodiments are shown in the drawing to better explain the idea underlying the invention. In the drawing, Figure 1 illus trates the invention applied to a transformer wherein the primary and secondary windings are fixed as regards their space relationship, and, 30 Figure 2 illustrates the application of the invention to an arrangement wherein at least one of the transformer windings may be varied spacially. Figure 1 shows an R. F. transformer in which the two coils I and 2 are positioned at a fixed 35 space relation to each other. These coils are supported by a bush 3 which possesses high electrical conductivity, and which is designed so that it will act upon such stray lines as may escape in the desired manner. Inasmuch as by the aid of 40 the said coil body or coil-former '3 having the action of a shield or screen, conditions are made so that the same flux will traverse both coils, the shape of the flux-lines outside the arrangement is quite immaterial. Still, it would be possible to 45 mount a protective shell which will surround the coils also from the outside so that the coils I and 2 in this instance are perfectly embedded,

as it were, within a conducting case.

In the embodiment shown in Figure 2 only 50 one of the two coils, i. e., coil 4 is fixed in space. The coil 5 is supported by a spindle 6, and by the aid of the latter, say, by revolving a small wheel or knob I, it is capable of being operated to different positions with reference to the coil 4. 55 Both coils are outwardly shielded by the aid of a cylindrical casing 8 which serves to preclude stray lines. It has been found that by the aid of such a cylinder, which, of course, should surround the coils as snugly as feasible on the outside in order to exercise an appreciable influence 5 upon the stray lines, the flux can be held or crowded together, as it were, to a great extent so that the maximum coupling that is attainable will have a high value. The coupling factor, of course, is considerably higher than when two coils 10 of the same sort are used in exactly the same position but without the outside shield. What is essential in this connection is that the shielding shell should be open at its ends to permit emergence of the flux lines. In this way the flux lines have an opportunity to form a closed loop in the outside space. If the casing were completely closed the flux lines would be compelled to form closed loops inside the shielded space, V and this would tend to defeat the very purpose that is aimed at here. The shape of the casing may be adapted to the conditions prevailing in a given case; indeed, -it need not necessarily be cylindrical. It is also possible to use prismatic shields of other cross-section, say, four-sided elements. The arrangement described with reference to Fig. 2 could be used also as a regulable inductance, and not only as a transformer or as a coupling system.

So far as the leakage between the coils and thus also the attainable coupling and the inductance are concerned, the relationship between coil diameter and coil length is .of importance. In the light of investigations it, has been shown that it is a suitable plan to choose the said relationship in such a way that the diameter will be about from two to four times the length.

Inasmuch as for the production of the desired effect high electric conductivity of the casing is a prerequisite, it is primarily copper that would 40 come into consideration for making the casing, and it is also a good plan to make the casing walls of adequate thickness seeing that, if the thickness of the walls were smaller, eddy-currents would not get a chance to develop in the desired manner in order to displace the stray lines.

Where the frequencies are extremely high, the wall thickness may be very small to make conditions adequate.

It is preferable not to make the shield of solid or compact material, but to subdivide or stratify it into constituent layers, or even to use a compressed dust which may be molded with insulation material.

It is known that not only the coupling, but

also other properties of the high-frequency transformer are governed to a great extent by the leakage. However, inasmuch as the leakage varies with the frequency, it may be said that said properties are a function of the frequency.

However, in the case of any set of prevailing conditions it will be feasible to find a solution which will satisfy to a large degree the requirement of frequency dependence. The subdivision of the shielding means as described constitutes a way particularly suited in the said respect.

I claim:

1. An air core radio frequency transformer including a hollow spool of material having high electrical conductivity shaped to provide a central hub portion with a radially outwardly extending flange at each end, and primary and secondary coils wound alongside each other and in close proximity to each other and to the hub and flanges of the spool, the coils and spool flanges being radially co-extensive.

2. An air core high frequency transformer including a hollow spool composed of compressed finely divided material having a high electrical conductivity, said spool being shaped to provide a central hub portion with a radially outwardly extending flange at each end, and primary and secondary coils wound alongside each other and in close proximity to each other and to the hub and flanges of the spool, the coils and spool flanges being radially co-extensive.

3. An air core high frequency transformer including a hollow spool made up of laminations of a material having high electrical conductivity, said spool being shaped so as to provide a central hub portion with a radially outwardly extending flange at each end, and primary and secondary coils wound alongside each other and in close proximity to each other and to the hub and flanges of the spool, the coil and spool flanges being radially co-extensive.

RUDOLF SCHMOOK. 

